Electrophoretic printing may be utilized with a group of electrodes in a printing apparatus for simultaneously imprinting a number of symbols or points upon a recording medium such as paper. Electrodes positioned adjacent the paper may be provided with specific shapes so that, upon the impressing of an electric field between the electrodes, electrically conductive particles of ink become attached to the paper with the particles of ink being arranged in accordance with the shapes of the individual electrodes. Thereby, a set of symbols may be simultaneously printed on paper. The ink is frequently carried to a location between the electrodes by means of a ribbon positioned adjacent the paper but spaced apart therefrom by a small distance on the order of the thickness of a few sheets of the paper.
A problem arises when the ink delivery by the ribbon is to be utilized for the imprinting of an image having various shades of gray in that the carbon particles utilized for producing the gray color, or black color, need be present on the ribbon with a prescribed density so that, upon the application of an electric field of a predetermined magnitude across the electrodes, a predetermined amount of the electrically conductive carbon is transferred by the electric field from the ribbon to paper. Indeed, for a precisely controlled gray scale of a printer or imaging system producing multiple shades of gray, a specific relationship is established between the magnitude of the electric field and the amount of carbon transferred from the ribbon to the paper. In the past, difficulties in the control of the gray scale have been apparent because of the lack of regulation of the density of the ink particles upon the ribbon. As a result, variations in the gray scale were obtained with variations in the density of the ink independently of the magnitude of the electric field.